Cellular mobile radiotelephone systems, a recently-developed technology useful for public mobile telephone service, can provide service quality and features comparable to those provided by the Public Switched Telephone Network (PSTN) for a large number of mobile users. Cellular radiotelephone systems are capable of using the same radio frequency (rf) communications channel for communications by multiple users (geographically separated from one another) independently and on a non-interfering basis, and thus use the portion of the radio frequency spectrum available for radiotelephone communications far more efficiently than ever before. However, the ever-increasing demand for radiotelephone communications service makes techniques permitting still more efficient use of rf spectrum and equipment resources extremely valuable.
Before cellular radiotelephone systems came into wide use, single high-power repeater stations were typically used to provide mobile-to-mobile and mobile-to-land communications. In such systems, a single repeater station is often located in a centralized position of high elevation within a predetermined geographical area to be serviced. All communications within the service area are routed directly through the repeater. The repeater typically provides full duplex communications simultaneously and independently over a plurality of fixed-frequency communications channels. Mobile stations within the repeater's service area can access the repeater to communicate with any other mobile station within the service area or to establish communications with a land line (i.e., conventional telephone line) coupled to the repeater. The repeater also is typically capable of calling a mobile station and initiating communications with the called station in response to control by a central dispatch operator or in response to signals received by the repeater from a land line. Fixed-frequency channels are allocated to calls using conventional trunking techniques such as those described in U.S. Pat. No. 4,360,927 to Bowen et al (1982) and U.S. Pat. No. 4,347,625 to Williams (1982). This type of communications system is extremely useful due to its flexibility and relatively low cost.
While systems of the type described above are very useful for servicing areas in which there are relatively few mobile stations or in which the users' demands are relatively low (e.g., due to the nature of the communications being conducted), system performance may be adversely affected as call traffic (volume) increases. The number of conversations capable of being sustained simultaneously with a single repeater-type system is equal to the number of repeater channels available for communications. Channel blocking can occur during periods of peak usage even using a repeater provided with a relatively large number of repeater channels (e.g., 20 or more), degrading system reliability and causing great frustration to users who attempt to initiate communications only to find that all channels are already in use. Allocation of additional repeater channels can reduce blocking potential but is sometimes not possible in congested metropolitan areas where many other services compete for available spectrum space.
Cellular mobile radiotelephone communications systems avoid the limitation of a one-to-one correspondence between the number of available channels and the number of independent conversations capable of being sustained simultaneously by re-using channels already in use in one portion of the service area on a non-interfering basis in another portion of the service area. Cellular systems divide a large service area into a number of smaller discrete geographical areas called cells (zones) each typically ranging in size from about 2 to about 20 kilometers in diameter. Each cell is contiguous with adjacent cells to provide continuous coverage throughout the service area. Each of the cells is served by a base station installation ("base station") including plural low-power transceivers which are capable of operating independently on different radio frequency (RF) channels. Each of the cell site base station installations is thus capable of participating in communication simultaneously with plural mobile radio transceivers operating within the associated cell. The cell site base stations also communicate via a data link (and voice trunks) with a central control station called a Mobile Telephone Exchange (MTX) the function of which is to selectively connect cell voice trunks to other cell voice trunks and/or to land lines and to coordinate activities of the cells.
FIG. 1 is a schematic representation of the geographical layout of a mature 13-cell cellular system 10. System 10 includes 13 cells C1 through C13 each having a cell site base station (T1-T13, respectively) associated therewith (each base station including at least one base station transceiver). System 10 may be considered an isolated piece of a larger system which has been fragmented. The contour or boundary of each cell is defined as a circle along which the received radio signal strength of signals transmitted by the cell site transceiver is equal to a predetermined minimum signal threshold. Such "bad service" contours characterize the minimum received signal strength indicator (RSSI) required to maintain adequate service within the cells.
Mobile radio transceivers within the contour of a cell can participate in communications of acceptable quality with the cell site base station associated with that cell (i.e., a minimum desired signal-to-noise ratio without excessive signal fading is maintained throughout the geographical area defined within the cell contour). The cell site contour may thus be considered dependent upon cell site transceiver effective radiated output power (erp) and mobile transceiver receiving sensitivity (as limited by cell site transceiver receiving sensitivity and mobile transceiver output power). Mobile transceiver output power may be selected so that the contour also corresponds to the signal strength of signals received by the cell site transceiver (i.e. the cell contours may also represent the area a transmitting mobile transceiver must be operating within for the cell site transceiver to receive the signals transmitted by the mobile transceiver at useful signal strengths). In this way, the range of the mobile transceivers may be made to correspond to the range of the cell site transceivers to avoid waste of resources and undue co-channel interference.
Cell site installations T1-T13 are positioned relative to one another such that there is a degree of overlap between contiguous cells. Hence, for instance, a mobile transceiver positioned in the area 15 of overlap between contiguous cells C5, C6 and C12 of system 10 could participate in communications with transceivers of any one of cell site base stations T5, T6 and T12. Some degree of overlap is important to permit transfer of calls from one cell to another, as will become more apparent shortly.
The great power and flexibility of cellular system 10 results in part because transceivers of cell site base stations T1-T13 may operate independently and on a non-interfering basis on the same RF channel (frequency). For instance, a mobile transceiver M1 located in cell C9 may communicate with a transceiver of cell site base station T9 via a communications channel X1 while, at exactly the same time, another mobile transceiver M2 located in another cell (for example cell C11) may communicate with a transceiver of associated cell site base station (T11) via the same communications channel X1 (i.e. the same frequencies are used for each call). Because cells C9 and C11 are geographically separated from one another (e.g., they do not overlap), the transmissions of mobile transceiver M1 and cell site base station T9 will not interfere with the communications between mobile transceiver M2 and cell site base station T11, and vice versa. The same channel X1 could also simultaneously be used in several other cells of system 10 so long as such other cells do not overlap the cells C9 and C11 in which the channel is already in use (for example, channel X1 might also simultaneously be used in cells C12, C13, C8, C1, C7 or C6, or even in combinations of such cells such as cells C8 and C1, cells C12 and C7, etc.). In this way, cellular system 10 is capable of handling many more independent conversations (calls) simultaneously than could a single repeater-type system allocated the same number of channels.
The coverage range and capacity of cellular system 10 is potentially unlimited. Additional cells may be added to increase the size of the area served by system 10. Moreover, existing cells can be split or sectored (e.g. by providing additional cell site transceivers coupled to omni-directional or directional antennas) to accommodate additional communications traffic within particular cells. The frequency reuse concept (whereby the same set of frequencies can be used virtually independently in non-contiguous cells) as well as the flexibility of accommodating increased traffic demands through cell splitting or cell sectoring has made cellular mobile radio systems the radiotelephone system of choice in North America as well as in Europe and Japan. The following references provide additional general background information concerning cellular radio systems and techniques:
U.S. Pat. No. 4,398,063 to Hass et al (1983); PA0 U.S. Pat. No. 4,308,429 to Kai et al (1981); PA0 U.S. Pat. No. 4,242,538 to Ito et al (1980); PA0 U.S. Pat. No. 4,127,744 to Yoshikawa et al (1978); PA0 U.S. Pat. No. 3,663,762 to Joel, Jr. (1972); PA0 U.S. Pat. No. 4,125,808 to Graham (1978); PA0 Brody et al, "Application of Digital Switching in a Cellular Mobile Radio System", International Switching Symposium (May 7-11, 1984, Florence, Italy); PA0 Ma et al, "DMS-MTX Turnkey System For Cellular Mobile Radio Application", Institute of Electrical and Electronic Engineers 1984 Vehicular Technology Conference (May 21-23, 1984, Pittsburgh, Pa.); PA0 Pandya et al, "Performance Modelling for An Automated Public Mobile Telephone System", International Communications Conference (June 13-17, 1982, Philadelphia); and PA0 EIA IS-3-B Interim Standard Cellular Mobile Station-Land Station Compatibility Specification (July, 1984).
Although some mobile transceivers served by system 10 may always remain in the same cell, it is generally desirable to provide continuous communications for mobile transceivers in transit between any two arbitrary points within the geographical area served by system 10. For instance, mobile transceiver M1 may belong to a business executive having a home located in cell C9 and an office located in cell C6 who wishes to use his or her mobile transceiver while commuting between home and the office. A conversation may be initiated while mobile transceiver M1 is in cell C9, but may continue as the mobile transceiver exits cell C9 and moves through, for example, cells C2, C1, C7 to finally reach a destination within cell C6. As mobile transceiver M1 approaches the "bad service" contour of cell C9, system 10 must somehow transfer ("hand-off") the ongoing communications to the new cell the mobile transceiver has entered or is about to enter (such as cell C2) if the conversation is to be continued. Overlapping of adjacent cells makes it possible for such hand-offs to occur without interrrupting the call being handed-off (since a mobile transceiver is typically in more than one cell at once when approaching the "bad service" contour of a cell). It is important that such hand-offs are accomplished very rapidly and reliably if conversations are to continue without interruption as mobile transceivers exit one cell and enter another.
A central mobile telephone exchange (MTX) (not shown) of system 10 supervises the cell site base stations T1-T13 to allow calls in progress to continue without interruption when mobile stations move from one cell to another. When a mobile transceiver participating in a call is about to exit a cell, the MTX automatically hands off the call to a free channel in an adjacent cell into which the mobile transceiver has moved (mobile transceivers can be in two or more cells simultaneously due to the overlap between cells). Cell site base stations typically continuously measure indication of received signal strength (RSSI) for each ongoing call and request the MTX to hand-off a call when the RSSI of the call falls below a predetermined threshold.
The cell to which the call is handed off may be selected in accordance with a voting process initiated by the MTX at the time the MTX is notified the RSSI in the transferor cell (the cell from which a hand-off is necessary) has dropped below the predetermined threshold. The MTX may at that time direct the cell site base stations of the cells adjacent to the cell serving the call (e.g., base stations T10, T3, T2 and T8 if cell C9 is serving the call) to monitor the strength of the signal transmitted by the mobile transceiver on the channel in use (monitoring receivers may be provided at each cell site solely for this monitoring purpose, or receivers of unused cell site transceivers may be used). The MTX typically receives the RSSI information back from the adjacent cells and orders cells by signal strength intensity. The MTX may select the cell in which the highest signal level was received and, if a service channel is available in that cell, the MTX may direct the mobile transceiver through the first cell to begin operating on that available service channel. If a service channel is not available in the cell determined as having the highest received signal level, other cells with received signal levels above a predetermined minimum threshold may be checked for available service channels until a new service
is located and allocated to the call. If no service channel is available after all possible cells have been checked, the call cannot be handed-off and the mobile transceiver may be signalled accordingly (in which case the mobile transceiver operator must either quickly complete his call or halt his vehicle before it exits the cell it is presently in).
A description of this intercell hand-off technique is found in, for example, U.S. Pat. No. 3,898,390 to Wells et al (1975). A variation on the Wells et al hand-off technique is disclosed in U.S. Pat. No. 4,475,010 to Huensch et al (1984), which describes a hand-off technique wherein cell sites themselves perform hand-offs in order to conserve MTX processing resources. When a signal from a specific mobile unit associated with a controlling cell site in the Huensch et al system drops below a prespecified threshold, the controlling cell site itself selects the group of nearby cell sites which are to measure the signal strength of transmission on the radio channel currently being used by the mobile unit. The MTX simply passes signal strength measurement request messages from the requesting cell site to each cell site in a list of nearby cell site addresses identified by the controlling cell site. The controlling cell site uses the signal strength reports and antenna/channel availability (also provided by the addressed cell sites) to generate a list of cell sites which are candidates for a hand-off. The controlling cell site further detects when the signal from a specific mobile unit drops below a second prespecified threshold, and selects a more limited list of nearby cell sites which are to measure received signal strength if this occurs.
As mentioned above, it may sometimes be desirable to direct a cell other than the one registering the highest received signal strength to handle a hand-off. For instance, U.S. Pat. No. 4,144,412 to Ito et al (1979) describes a cellular radio system having overlapping cells. When communications is to be established with a mobile station, the mobile station transmits a call signal over a control channel receivable by each of the cell site transceivers. The installations at each cell site measure the call signal strength, and the cells are ordered in accordance with received signal strength. A search is then made to determine if the cell site base station which received the signal of maximum intensity has a channel free to handle a call. If such an idle channel exists, information designating the idle channel is transmitted to the mobile station and communication is established between the cell receiving the highest signal strength and the mobile station. On the other hand, if the search for an idle channel reveals that the cell site base station which received the signal having the maximum intensity has no available channels, it is determined whether the cell site base station receiving the next strongest signal intensity has a channel available for communications. If this second cell site installation likewise has no available channels, a third cell site base station having the next highest received signal strength is checked to determine if it has an available channel. The number of searches and designations to be repeated is determined in accordance with the intensity of the received signal, the degree of congestion of the communications and other conditions.
A vehicle in a congested cell may thus be included in an adjacent cell by designating a communications channel of a cell which has received a signal having an intensity next to maximum, with the result that the equivalent area of the congested cell is effectively narrowed and the area of the cell adjacent to the congested cell is effectively increased. Even when the channels of a cell site in which a calling vehicle is located are all busy, it is possible to complete a call by using an idle channel assigned to an adjacent cell, thereby decreasing the number of call failures, increasing the quantity of traffic which can be handled, and improving the efficiency of channel utilization.
U.S. Pat. No. 4,435,840 to Kojima et al (1984) discloses a somewhat similar technique in which cell size is not merely effectively, but actually selectively enlarged or reduced (e.g., by controlling the output power of the cell site transceivers) in accordance with a value representing current or instantaneous volume of traffic handled by the cell (i.e., a utilization factor of equipment of each cell site base station). If the traffic in a first cell is much greater than the traffic in an adjacent cell, the size of the first cell is reduced and the size of the second cell is increased (by adjusting cell site transceiver and/or mobile transceiver power outputs and/or receiver sensitivities) to permit the adjacent cell to handle traffic which the first cell would otherwise have been expected to handle.
U.S. Pat. No. 4,144,496 issued to Cunningham et al (1979) discloses an adaptive channel assignment technique for use in a cellular radio system having cell site transceivers which are remotely tunable by the MTX. Each cell site transceiver may be selectively tuned to any one of the frequencies allocated to the system under MTX control. Additional transceivers are provided at cell sites to permit shifting of channels from cell to cell as needed to accommodate heavier traffic loading in busier cells by diverting channels from lightly loaded cells. An algorithm designed to minimize interference is used to control the channel assignments.
U.S. Pat. No. 3,764,915 to Cox et al (1973) is also of interest in disclosing a cellular radio system wherein an MTX dynamically allocates communication channels in response to requests for channels by mobile users. The determination of the channel which should be allocated is made in accordance with channel re-use and allocation optimization criteria to insure that it is the preferred allocation from the standpoint of desired system performance. More particularly, channels are allocated to cells on the basis of close spatial proximity to the requesting mobile transceiver, relative velocity of the mobile transceiver, etc. in order to prevent co-channel interference and to avoid "wasting" channels by uneconomically assigning them.